Mechanical mixers typically used in the polymer and rubber industries primarily rely on shear as the primary mode of dispersive deformation. In these types of dispersive mixers, rotors or stirrers are typically used to disperse ingredients within the primary matrix. The majority of mixing processes known in the art, such as those done with Banbury batch mixers and twin screw mixers employ the rotating action of a screw or rotors as the major energy input to the system to mix materials. Because of such rotating action, these processes subject materials to a high level of shear flows as opposed to extensional flows. The high levels of shear flows in these processes have two results. First, such processes are relatively energy inefficient, and shear flows contribute to significant energy dissipation and rapid heat rise in the system being mixed, thus often limiting the amount of mixing that is possible without degrading the material. Mixing processes that rely on simple shear flow require several orders of magnitude more energy than those that rely on extensional flow to achieve the same level of mixing. Second, the dispersive mixing capability based on the shear flow component of these mixing processes is poor. It is known that extensional flows, on the other hand, are much better at breaking up inhomogeneities in a material than shear flows and are therefore much better at dispersive mixing. In fact, it is the extensional flow capabilities of rotation-type mixing devices that contribute most effectively to the limited success of these devices as dispersive mixers.
U.S. Pat. No. 5,451,106 discloses an extensional flow mixer containing a series of chambers that are separated by several complex convergent/divergent surfaces providing narrow openings between the chambers. The overall direction of flow in the disclosed device is radial as opposed to axial.
U.S. Pat. Nos. 5,904,422 and 5,749,653 disclose continuous squeeze flow mixers in which a reciprocating motion translating member is contained within a chamber with inlet and outlet ports to provide dispersive deformation in the form of a squeezing type flow over a disc contained on the translating member. Although the primary mode of dispersive deformation is biaxial extension, energy dissipation in the form of friction can still occur as a result of slip over the squeezing surface.
U.S. Pat. No. 5,968,018 describes very low viscosity fluids or gasses being forced through several staggered arrays of static orifice plates in order to achieve fluid atomization and/or turbulent mixing associated with high Reynolds number flows. By contrast, the flow associated with polymeric fluids is inherently laminar (non-turbulent) having a low Reynolds number flow. Consequently, the described devices are not pertinent to the mixing of high viscosity polymeric fluids and highly filled polymeric fluids.
U.S. Pat. No. 3,860,218 discloses an apparatus comprising a nozzle block in the form of at least one and preferably a plurality of parallel ducts clamped between a pair of pressure cylinders, between which substances are pushed from one side of the block, through the block to the other. The treatment of a single batch is accomplished by moving the material back and forth between the cylinders.